既能用于标记蛋白质氨基，又能标记蛋白质醛基的活化生物素是()
A．HZ
B．BCHZ
C．MPB
D．BNHS
E．BCNHS
参考答案：B
Tom didn't go to school _____ this morning because he overslept. [ ]
A. at once
B. on time
C. by accident
D. at present
选用焊条时，应考虑被焊工件的物理性能、化学性能、工件条件、施工条件、生产效率及经济性等因素。( )
